The present invention relates to a setter which is used in sintering and which has an oxide coating layer, which is used upon sintering components, and a production method thereof, and further relates to a refractory metal plate having an oxide coating layer, and a production method thereof.
In recent years, production of iron series, copper series, and tungsten series processing objects and components by means of metal injection molding (hereinafter referred to as “MIM”) has been put to practical use and, following it, functional demands to a setter used in sintering have been enhanced.
Conventionally, high temperature resistant materials, such as Al2O3 (hereinafter referred to as “alumina”) and SiO2 (hereinafter referred to as “silica”), have been often used for the setter used in sintering.
However, in case of the high temperature resistant material, such as alumina or silica, thickness of the plates should be set to, for example, 10 to 15 mm for proof thermal shock or deformation due to weight of processing objects. Herein, the processing object may be an object to be treated by sintering or heating. On the other hand, when this thick high temperature resistant plate is used, the loading/sintering amount of the objects is limited, and further, enormous energy is required for raising the temperature of a furnace upon sintering, long time is required for lowering the temperature because of the plate's small thermal conductivity.
For solving them, such a setter used in sintering has been demanded that has a less thickness to enable increase of the loading volume of the processing objects, and further, that still maintains the characteristic of the conventional high temperature creep resistance plate.
A plate is made of a refractory metal, such as molybdenum or tungsten, so that the plate is excellent in characteristic of high temperature creep resistance.
As a plate having heat resistance, a molybdenum plate has been proposed in JP-A-S61-143548, JP-A-S63-157832, and JP-A-S63-192850, which will be hereinafter referred to as reference 1, reference 2, and reference 3, respectively. The reference 1 discloses a molybdenum plate made of a pure molybdenum metal added with no dopant, having a size of a disk surface being 15 mm to 150 mm, and provided with crystal grains accounting for ⅕ or more of a thickness in a thickness direction of the plate.
On the other hand, the references 2 and 3 each disclose a molybdenum plate which contains lanthanum oxides arranged in a direction substantially perpendicular to a thickness direction of the plate and, particularly, the reference 3 discloses the molybdenum plate wherein crystal grains exhibit an interlocking structure.
However, when the bare molybdenum plate is used while being brought in contact with MIMed products for sintering thereof, the MIM products being processed are melted and adhered to the surface of the molybdenum plate so that the yield of the sintered products is extremely poor.
In view of this, a molybdenum plate provided with an adhesion preventing layer on the surface thereof is proposed in, for example, JP-A-2002-47581 and JP-B-2764085, which will be hereinafter referred to as references 4 and 5, respectively. The reference 4 discloses that a molybdenum plate doped with lanthanum or lanthanum oxides is buried in powders of a mixture of at least one of aluminum, chromium, and titanium, and alumina to perform a reduction heat treatment to thereby diffuse metal elements into the molybdenum plate from the surface, then a heat treatment is applied thereto in an oxidization atmosphere so that an oxide layer is formed on the surface thereof as the adhesion preventing layer.
On the other hand, the reference 5 discloses that, by plasma spraying molybdenum powder and then alumina powder according to a method of plasma spraying of ceramics, an alumina layer is formed on the surface of a pure molybdenum plate via a composite layer of molybdenum and alumina.
JP-A-2000-516666, which will be hereinafter referred to as reference 6, discloses a parent substance consisting of refractory metals and an oxidation protective coating made of silicides or aluminides. In the parent substance, a reaction barrier layer is formed between the substance and the oxidation protective coating by means of plasma spraying.
Conventionally, there have been a case where the high temperature resistant material such as alumina or silica is used for a plate that is used upon sintering iron series, copper series, or tungsten series objects or components produced by MIM or the like, and a case where the high temperature resistant material such as molybdenum or tungsten is used for such a plate.
In the former case where the high temperature resistant material, such as alumina or silica is used, a thickness of the plate should be set to, for example, 10 to 15 mm for proof thermal shock or deformation due to weight of processing objects. Consequently, there has been a problem that when the thickness of the plate is large, charge amounts of the processing objects are reduced, much energy is required for raising the temperature upon sintering, and further, it takes long time to cool it because of its small thermal conductivity and large specific heat.
In the latter case, since the processing objects and the plate adhere to each other upon sintering, alumina or the like in the form of powder or sheets is interposed therebetween. However, the alumina powder or the like adheres to the processing objects by adhere so that much labor is required for remove before and after the sintering process.
Further, when heated up to 500° C. or higher in the oxidization atmosphere, the molybdenum plate is extremely oxidized and sublimed, therefore, can not be used for sintering in the air.
As disclosed in the references 4 and 5, it has been proposed to form the oxide layer or the ceramic layer on the surface of the molybdenum plate for the purpose of preventing the melting adhesion of the processing objects. However, the formation process is complicated and laborsome.
When molybdenum is present in the uppermost layer of a plurality of surface layers, the MIM products are subjected to the melting adhesion thereto. Further, inasmuch as the layer containing molybdenum is plasma spraying as an underlayer, even if the uppermost layer does not contain molybdenum, molybdenum is liable to enter the outermost surface due to diffusion or the like so that there arises an instance where the melting adhesion between the MIM products and molybdenum setter can not be prevented.